The recording of the DLNP seminar “Magnetic Shielding of Large Photoelectron Multipliers for the OSIRIS Facility of the JUNO Detector” given by Oleg Smirnov on December 7, 2022.

Large (20") PMTs used in the OSIRIS facility of the JUNO detector are sensitive to the Earth’s magnetic field (EMF). There is no available space for installation of EMF compensation coils at the location of the OSIRIS facility, and PMTs of the detector will operate in the non-compensated EMF. The technical design and characteristics of the magnetic shield developed for the OSIRIS facility were presented at the seminar. A ribbon of amorphous alloy with the extreme magnetic permeability was used for its manufacture providing the excellent efficiency of the Earth’s magnetic field screening with relatively small amounts of the material. The mass of materials is crucial when constructing low-background facilities from the point of view of radioactive backgrounds. The use of the amorphous material is cost-efficient compared to other methods of the Earth’s magnetic field screening. In total, 77 screens were prepared and delivered to the collaboration last year.